Phone polls are done by randomly calling numbers and asking if they'd like to take part until you've got enough to call it a representative sample (usually between 1000-2000 responses). With online polls, for YouGov at least you have to sign up to their panel and they'll feed you surveys but you might never get a Westminster voting intention one.
If you think about there being, say, 150 polls in a year, averaging 1,500 responses (and assuming there's no overlap among these people), that still only comes out to 225,000 people - with an electorate of 46.5m as of the 2016 referendum, it's very unlikely any of the vast majority have been individually polled. Ask enough random people on the street though and the chances are a fair few will have been.